The invention relates generally to overvoltage protection equipment for electrical apparatus and particularly to that useful for series capacitor installations in high voltage alternating current transmission lines.
Reference is made to U.S. Pat. No. 3,889,158, June 10, 1975, by the present inventor whose description is incorporated herein by reference in its entirety and which sets forth an invention upon which the present invention is an improvement.
The prior patent provides an arrangement of protection equipment that goes beyond what was previously used for capacitor protection in that the series capacitor bank can be bypassed at an overvoltage level that is not high enough to present any danger to the series capacitors themselves, but which is of a magnitude that may give rise to system transients producing oscillating shocks on the rotating shafts of generation equipment coupled to the transmission line. It is desirable that this protection level be as low as possible but conflicting requirements are encountered because a lower voltage protection level means the capacitors are removed from the system more readily and more frequently, during which time they are not performing their intended function, and they have to be restored rapidly after fault clearing. The reinsertion problem was addressed in the prior patent and solved by having a "dual sparkover" feature, namely a pair of trigger gaps of different voltage level. When the more sensitive trigger gap fires it causes the other trigger gap and the main spark gap also to fire, upon which the circuit of the most sensitive trigger gap is opened and it is disabled for a period of time to permit clearing of post reinsertion transients. While the first trigger gap is disabled, the second, less sensitive, trigger gap is in the system and provides protection against any overvoltage that could damage the capacitors.
Such a system can be practically implemented where the overvoltage levels of the two trigger gaps have a limited difference. For example, the specific example of the prior patent permits firing the first trigger gap at about 2.0 to 2.5 per unit (P.U.), i.e., 2.0 to 2.5 times the normal rated capacitor voltage level, while the second trigger gap is fired at a sparkover level of about 3.5 P.U. and the main spark gap, a larger current capacity gap, normally a carbon gap, has a sparkover of about 4 P.U. At such levels the system can be successfully operated. However, it is possible that before the first trigger gap goes at 2.0 to 2.5 P.U. there could be harmful transients produced that endanger the generating equipment. It is hence desirable to provide a scheme which permits removal of the capacitors at a lower overvoltage consistent with other system requirements including effective firing of the main spark gap and effective rapid reinsertion of the capacitors.
Reference is also made to copending application Ser. No. 767,185, filed Feb. 9, 1977, by the present inventor and assigned to the present assignee, now U.S. Pat. No. 4,121,270, issued Oct. 17, 1978, which provides one form of solution to the foregoing problem. In the pending application instead of a dual sparkover system there is provided a single bypass spark gap with means for force firing the spark gap upon the occurrence of any of a variety of system conditions that could result in subsynchronous oscillation. The force firing circuit includes a capacitive potential device supplying power to energize a firing control circuit with a pulse transformer placing high frequency voltage across the protective device or spark gap for rapid bypassing and also rapid reinsertion of the series capacitor bank. While such equipment is effective and provides a degree of flexibility to satisfy varying system performance conditions, it also requires greater complexity of equipment and higher cost than a dual sparkover scheme.
The present invention came about as a result of efforts to improve upon the dual sparkover scheme of the aforementioned issued patent.